On compression now keep in mind that spinning over with the rope is static compression .Running under power is dynamic which will be higher .Type of piston rings .
My 2100 Husqvarna which is a thick ring pulls over about like a Harley on the rope .Yet most 2100 XP's with the thin ring pull over fairly easy but they will out run mine .
ECopsey had the other 2100 and I never asked him what it pulled static and I really have no idea what mine pulls other than hard but that's the way a thin ring does .If I am correct the way they are made under power it blows the ring outward making a better seal .--just trivia might mean something might not .
Yes, the engine speed will have a large effect on the pressure in the combustion chamber. I wouldn't use static and dynamic to describe the states, because static implies no motion or an idealized calculation. Also because using these adjectives with cylinder pressure can be make definitions of Static vs. Dynamic Compression Ratio confusing.
Static Compression Ratio (SCR) is defined as the full swept volume. Dynamic Compression Ratio (DCR) in a four-stroke is the volume after the intake valve closes, in a two-stroke is when the export closes. This is the same as the Corrected Compression Ratio which is more often used in reference to two-strokes. Even then, the DCR or CCR are an idealized, maximum compression assuming 100% ring seal. It also assumes that the rest of the swept volume is lost out the ex port without effecting the pressure in the cylinder... which is not the case.
Thin ring saws have a tendency to pull over easier because of the large ring gap that looses a lot of air compared to the smaller gap of the thick ring. At running velocities the loss of air through this small orifice is negligible. Also thin rings have significantly less ring tension and therefor frictional losses which will also make it feel like it pulls over easier.
All pistons are designed to leak some of the gas pressure of the cylinder behind the rings, pushing them into the cylinder. This is because the ring is riding on the lower surface of the ring groove allowing a slight gap above the ring where the gas pressure can get behind the ring. Some piston features (gas porting), are aimed at letting the pressure build behind the ring sooner and I have used them in a chainsaw before. It becomes more of benefit with fuels that needs lots of compression to obtain the maximum power such as alcohol or methanol. For chainsaws I usually use horizontal gas porting, on kart engines I'll vertically port them.
Other things can be done to a four-stroke engine to increase ring seal. One way is to decrease ring flutter, or the short time period where the ring is off the lower portion of the ring groove after TDC before the combustion gasses have created enough pressure to force the ring down. Often this is done by creating a vacuum in the crankcase to keep the ring attached to the lower wall of the ring groove. It's common to see over 20 inHg of vacuum in the crankcase in a dry sump racing engine.
